This section provides background information related to the present disclosure which is not necessarily prior art.
Shock absorbers are used in conjunction with automotive suspension systems and other suspension systems to absorb unwanted vibrations which occur during movement of the suspension system. In order to absorb these unwanted vibrations, automotive shock absorbers are generally connected between the sprung (body) and the unsprung (suspension/chassis) masses of the vehicle.
Various types of shock absorbers have been developed to generate the desired damping forces in relation to various vehicle performance characteristics. For example, a piston located within a pressure tube of the shock absorber, is connected to the sprung portion of the automobile through a piston rod and the pressure tube is connected to the unsprung portion of the automobile. The piston is able to limit the flow of fluid between opposite sides of the piston through valving. Specifically, when the shock absorber is compressed or extended, the shock absorber is able to produce a damping force which counteracts the unwanted vibration which would otherwise be transmitted from the unsprung portion to the sprung portion of the automobile.
In another example, a dual tube shock absorber has a fluid reservoir defined between the pressure tube and a reservoir tube. A base valve located between the lower working chamber (the area below the piston) and the reservoir limits the flow of fluid between the lower working chamber and the reservoir to produce a damping force which also counteracts the unwanted vibration. The greater the degree to which the flow of fluid within the shock absorber is restricted by the piston and/or the base valve, the greater the damping forces which are generated by the shock absorber. Thus, a highly restricted flow of fluid would produce a firm ride while a less restricted flow of fluid would produce a soft ride.
Due to an exponential relationship between pressure drop and flow rate, it is difficult to obtain a damping force at relatively low piston velocities, particularly at velocities near zero, while still maintaining acceptable damping forces at the relatively high piston velocities. A low speed damping force is important to vehicle handling since most vehicle handling events are controlled by relatively low speed vehicle body velocities which thus lead to relatively low speed piston velocities.
Various systems for tuning shock absorbers during low speed movement of the piston include a fixed low speed bleed orifice which is always open across the piston and/or the base valve. The bleed orifice can be created by utilizing notches positioned, for example, on a flexible disc adjacent to the sealing land. In order to obtain the low speed control, the notches have to be small enough to create a restriction at relatively low velocities. When this is accomplished, the flow of fluid at low speeds of the valving system operates over a very small range of velocity.
As the temperature of the fluid increases or decreases, the consistency of the fluid also changes. For example, the viscosity of the fluid is higher (thick fluid) at cold temperatures and is low (thin fluid) at high temperatures. Accordingly, at high temperatures, the bleed orifice having a straight channel may not restrict the fluid flowing and at low temperatures the fluid may cause the discs to deflect prematurely, thereby affecting the damping characteristics of the shock absorber at low piston speeds.